Selenium tablet and method of making and applying it



July 2, 1946. o. sAsLAw 2,403,026

SELENIUM TABLET AND'METHOD OF MAKING AND APPLYING IT Filed April 24, 1942 INVENTOR 0770 S sLAW Patented July 2, 1 946 SELENIUM TABLET AND METHOD OF MAKING AND APPLYING IT Otto Saslaw, Kearny, N. J., assignor to Federal Telephone & Radio Corporation, a corporation of Delaware Application April 24, 1942, Serial No. 440,411

6 Claims. 1

This' invention relates to selenium wafers-or tablets and methods of making them and of applying them to a base plate to form selenium elements.

The object of this invention is to provide a simple method of making a selenium wafer in a form from which it can readily be applied to a base plate.

In the making of selenium discs heretofore it has been customary to apply amorphous selenium powder, preferably in the vitreous form, on a base plate, heat the. plate to fuse the selenium, and spread the selenium over the plate while it is being fused. Considerable time has been taken in spreading the fused powder, owing in large part to the high surface tension of molten selenium and also the fact that air bubbles must be removed. The length of time required to coat the hot plate has the disadvantage that those parts of the plate which have not yet received the selenium readily oxidize which tends to injure'the final product.

In accordance with my invention, I provide the selenium in a wafer or tablet form and preferably make the tablet of the proper size to cover the base plate, so that by placing the wafer or tablet on the base plate, which is heated, all parts of the surface of the base plate are simultaneously covered with the selenium and the fusing takes place simultaneously over all parts of the surface. This facilitates the application of the selenium to the plate and minimizes the undesired oxidation of part of the plate which might occur if such parts were left uncovered for a time while hot.

In the drawing:

Fig. 1 illustrates the pressure plates of a press in which the wafer may be made;

Fi 2 is a face view of a wafer made according to this invention;

Fig. 3 is a side view of the wafer;

2-. ments may be placed in these wells to heat up the heads while the pressure is being applied. The actual surfaces between which the selenium wafer is to be pressed should be of a material which is not injurious to the selenium, and for this purpose mica and aluminum have been found satisfactory. The plates 6 and I are accordingly preferably made or faced with mica or aluminum. A

pressure which I have found satisfactory is several thousand pounds per square inch and I have found 120 C. a proper temperature to use at this pressure. It will be understood that the permissible pressure and temperature can vary considerably from these-particular values however. In actual practice, the amount of pressure and the temperature will bear a proper relation-to pressed. The shape of the selenium wafer need not necessarily be circular as shown in Fig. 2, but might be made square or rectangular or some other shape, as desired, the shape being deter-,

mined somewhat by the shape in which the powder isfirst applied between the pressure plate. For example, if a circular wafer is desired, the powder should bev piled evenly in a pile between the pressure plates, so that whenxwarmed and pressed it will spread out evenly in all directions into the circular shape. Such a circular finished tablet is shownin Figs. 2mm 3, which will be removed from the press as a solid, unbending wafer.

40 The application of this wafer to a base plate Fig. 4 illustrates a base plate on which the wafer has been placed; and

Fig. 5 is a cross-sectional view of the plate and wafer 01' Fig. .4.

According to this-invention, a measured quantity of selenium, in the amount desired tov be placed on the plate to be coated, is compressed between the pressure plate of a press. This is illustrated in Fig. 1 wherein the selenium in the so well known black vitreous powder form is placed between the pressure heads! and 3 of the press.

the rest of the press mechanism not being shown. v

is illustrated in Figs. 4 and 5, wherein-the wafer 8 is shown placed upon the base plate I which will ordinarily have in it a central hole II. The

ture above the melting point of the selenium to melt the wafer. H iormly throughout the surface of the plate and will be" rapid enough so that very little oxidation will form on the plate prior to the melting and fusing of the selenium. As is well known, if such a molten layer of selenium be allowed to cool down to about room temperature itsolidifies, forming a vitreous non metallic coating. which could be converted, if desired,- to the metallic upon the amount ofthe selenium powder com- 1 base plate should be heated up to a tempera- The meltin will occur uniform by heat treating at a temperature somewhat below the melting point of selenium.

In this fusing operation a slight amount of touching up of the molten selenium to the plate may be required to compensate for the surface tension shrinkage of th'e'selenium, in order to obtain a uniform coverage and fusing. a

The use of such a selenium wafer and the method of application to the base plate is advantageous both from the point of view of simplicity the method of coating 9. base plate with selenium,

and of advantageous effect on the plate. The

method is simple as the coating operation re wafer or tablet is to convert the tablet of vitreous amorphous selenium, taken from the press, into the gray metallic state before applying it to' the base plate, Thi may be done by taking the vitreous tablet shown in Figs. 2 and 3 and heating it at a temperature of from 170 C. to 217 C. A temperature of 200 C. is found highly advantageous. This high temperature heating should be continued until the black color of the vitreous form of tablet is converted to the gray metallic color. It has been found that the gray metallic tablet covers the base plate surface more evenly than does the vitreous tim of tablet upon melting.

If the tablet is made slightly oversize with respect to the base plate, it will be found that upon melting, the tablet will assume almost the exact shape of the base plate contactsurface. Hence, it is advantageous to make the tablet somewhat oversize if complete coverage is desired. It will also be found that the surface tension of the selenium will act to'cause it to uncover the central hole I. which is usually pressed in the base plate or cathode 9 of the selenium rectifier. The

reason for this is that the shrinkageof the tablet -during the heating and fusing at the position over the hole whereit is unattached to the base plate causes a corresponding hole to open up through the selenium as the selenium over the hole is drawn by its surface tension toward the base plate.

It should be understood that the invention is not necessarily limited to the making or useof vitreous or metallic selenium wafers, as it is nossible to make the Waters of .red amorphous selenium powder in substantially the same way as in thecase of the vitreous wafers. Theme of the vitreous form instead of red form is usually preferable for use in rectiflers, as the vitreous form has been found to give the. better results.

References herein to selenium elements are intended to cover selenium rectiflers and selenium photo-electriccells.

What is claimed is: 1. In the manufacture of selenium elements,

which comprises completely covering the surface 'of the base plate to be'coated with a wafer of compressed selenium powder, and heating the plate to a temperature suilicient to fuse the selenium to the surface.

2. In the manufacture of selenium elements,

the method of coating a base plate with selenium,

which comprises completely covering the surface of the base plate to be coated with a wafer of compressed selenium powder, and then heating the plate to a temperature above the'melting point of selenium to melt the wafer and fuse the selenium to the surface.

3.- The method of making a selenium element,

Y which comprises completely covering the surface .ing the base plate of a base plate with a thin cohesive selenium wafer, and heating the base plate to melt the selenium.

4. The method of making a selenium element,

which comprises compressing amorphous selenium powder under high pressure and at an elevated temperature in the general order of about 120 C. to form a cohesive wafer, then completely covering with said wafer the surface 'of a base plate to be coated with the selenium, and heatto fuse the selenium to the surface.

5. The method of making a selenium element, which comprises making a wafer by compressing.

' which comprises compressing amorphous selenium powder at a temperature in the general neighborhood of about C. and a pressure of the general order of several thousand pounds per square inch, to form a selenium wafer, then converting thewafer to the metallic form by heating it at a temperature of the general order of about C. to 217f 0., then completely covering by the wafer the surface of a base plate to be coated,

" and then fusing the metallic wafer to the plate buy heating the plate to the fusing point of sele- O'I'l'O SASLAW. 

